Middle Eocene dyke with columnar divisions (entablature).
Columnar jointing produces some of the most stunning scenic aspects of lava flows and other volcanic deposits. Columnar jointing consists of sets of regularly spaced parallel fractures (joints) that intersect in a roughly prismatic pattern. Most columns are hexagonal, but they may have anywhere from three to seven sides.
close up of columnar basalt
Columnar jointing is most commonly found in basaltic lava flows, but it also occurs in lava flows of other compositions, pyroclastic flow deposits (ignimbrites), in some lava domes, volcanic necks, and even in some shallow intrusions.
Devils Postpile National Monument and Devils Tower National Monument contain the best-known examples of columnar jointing in the United States. Devils Postpile NM in California was established primarily for the spectacular columnar jointed lava flow that makes up the heart of this small national monument. Erosion along columnar joints produces the nearly vertical columns that make Devils Tower in Wyoming so distinctive and compelling.
